This invention relates to a method and an apparatus for anti-skid control in which wheels are prevented from being locked upon being braked.
In a known apparatus for anti-skid control for preventing wheels from being locked upon being braked, while an anti-skid control is effected, a modulator reduces the brake fluid pressure in a wheel cylinder and thereafter increases it again under certain conditions so that the braking torque (total braking torque including braking system torque) is increased linearly with respect to time. In this pressure re-increase state, the anti-skid control is achieved in the following manner for example. When it is determined from the information given by a wheel speed detector that the wheel acceleration exceeds a predetermined threshold in a zone where the wheel speed is decreasing, it is assumed that the rate of change k of the coefficient of road surface friction with respect to the wheel slip rate is in a range giving a stable braking condition represented by k&gt;0 and the pressure re-increase state by means of the modulator is maintained. When the rate of change k comes below the threshold, it is judged that the rate of change is in a range giving an unstable braking condition represented by k&lt;0 and the brake fluid pressure is reduced by the modulator.
In this specification, the term "braking torque" means a total torque to effect braking of a wheel including a brake torque created by a braking system and a torque created by the engine-brake effect and the like. The term "braking system torque" means a torque created by a braking system.
Incidentally, the inventors have invented a method for calculating the rate of change k from the wheel speed detected at four points and other factors as an improvement of the anti-skid control apparatus noted above. The improved method has been applied for a patent in Japan and it has been disclosed in Japanese Patent Public Disclosure No. 7-81542.
In the prior art method disclosed in this Japanese Patent Public Disclosure, since the rate of change k of the coefficient of road surface friction with respect to the wheel slip rate must be calculated from values of the wheel speed detected at four points, the process time for the calculation is relatively long and therefore, there is a need for further improvement.